1. Field of the Invention
The invention relates in general to a thermoelectric device with thin film elements and stacks having the same, and more particularly to the thermoelectric device with thin film elements and stacks having the same with highly thermoelectric efficiency of power conversion for cooling/heating and electricity generation.
2. Description of the Related Art
A thermoelectric device is a module with properties of direct conversion of temperature differences to electric voltage and vice versa. Due to this property of thermoelectric conversion, both cooling/heating and power conversion applications are possible with emerging thermoelectric technologies. For example, this thermoelectric effect can be used to generate electricity, to measure temperature, to cool/heat objects. Simply put, a thermoelectric device can cool/heat an attached object when a current is flown in an appropriate direction through the thermoelectric material, and it is applicable in the field requiring cooling/heating technologies. Further, a thermoelectric device can produce electrical energy when types of thermoelectric material with a temperature difference between two sides, and it is applicable in the field of generating electricity.
FIG. 1 is a cross-sectional view of a conventional thermoelectric apparatus. The conventional thermoelectric apparatus of FIG. 1 is manufactured using two electricity-insulated substrates 121a/121b with the conductive metal layers 111a/111b, a P-type thermoelectric block 101 and an N-type thermoelectric block 102 sandwiched between them. The efficiency of thermoelectric power conversion is mostly determined by the properties of the P-type thermoelectric block 101 and the N-type thermoelectric block 102. As shown in FIG. 1, the upright P-type thermoelectric block 101 and the N-type thermoelectric block 102 are arranged electrically in series using the conductive metal layers 111a/111b. The electricity-insulated substrates 121a/121b could be made of ceramic material for adding rigidity and the necessary electrical insulation. The material of N-type thermoelectric block 102 has an excess of electrons, while the material of P-type thermoelectric block 101 has a deficit of electrons. One P and one N make up a couple, and a thermoelectric module can contain one to several hundred couples. Take a thermoelectric cooler/heater application as example. When a voltage is applied, a current flows in the P-type and N-type thermoelectric blocks 101/102 upward or downward, and a different temperature on each side of the substrate is created, which the direction of current flow is parallel to the direction of heat transfer. The temperature difference created by the thermoelectric device can be used for cooling or heating an object. In the application of electricity generator by using temperature difference, the direction of heat transfer is still parallel to the direction of current flow. However, the thermoelectric power conversion of this conventional thermoelectric apparatus is inefficient due to the limitation of bulky thermoelectric material property with low Figure of Merit (ZT) value. Typically, the maximum cooling capacity of the conventional thermoelectric apparatus is about 3˜5 W/cm2, and the electricity generating efficiency is about 2˜3% when a temperature difference of 200° C. is applied to the cool and heat ends. It would be a direct and effective method of increasing the efficiency of thermoelectric power conversion by constructing the thermoelectric materials with high ZT value into the device.
Many experiments studying thermoelectric materials and researches attempting to improve performance of thermoelectric devices have been underway for the past 20-30 years with not much success. Workers in the thermoelectric industry have tried to make a breakthrough in the reported ZT value=1. The value of the figure of merit, commonly expressed as the dimensionless figure of merit ZT, is usually proportional to the efficiency of the device. In 1993, Hicks and Dresselhaus et al (MIT professors) have published the articles that project theoretically very high ZT value as the thickness of thermoelectric material are made progressively thinner (ex: nano-dimension). In 2001, Venkatasubramanian et al. (Research Triangle Institute, United State) have discovered a superlattice of thin film of P-type Bi2Te3/Sb2Te3 with a ZT value of about 2.4 at room temperature, which breakthrough the bottleneck of ZT value of 1. In 2004, Hi-Z Technology, Inc. (San Diego of United State) studied a quantum well thin film of P-type B4C/B9C and N-type Si/SiGe, and has claimed a ZT value lager than 3 can definitely be achieved. Accordingly, the thermoelectric thin films to date are known to be good at high ZT value, and definitely superior to the conventional bulky thermoelectric material. When the thermoelectric thin films are used for fabricating the thermoelectric device, the efficiency of thermoelectric power conversion can be greatly increased. Additionally, the thermoelectric thin films are especially suitable for manufacturing the thermoelectric micro-devices since less material (i.e. only layers of thin film) is required. Thus, the device with thermoelectric thin films is a potential star in the development of the related fields such as the applications of micro-cooling and thermoelectric generator.
However, the performance of a conventional device having thermoelectric thin films constructed based on a typical semiconductor is not very well. FIG. 2 is a cross-sectional view of a conventional device having thermoelectric thin films. As shown in FIG. 2, a P-type thermoelectric thin film 201 and an N-type thermoelectric thin film 202 are disposed between two much thicker substrates (i.e. an upper substrate 221a and a lower substrate 221b). Also, the conventional device further includes the metal pillars 231 and the conductive metal layers 211a/211b, which are disposed between the thermoelectric thin films 201/202 and the upper/lower substrates 221a/221b. The P-type thermoelectric thin film 201 and the N-type thermoelectric thin film 202 are adhered to the upper substrate 221a through the soldering layer 241, and the metal pillars 231 are adhered to the lower substrate 221b through the soldering layer 242.
Although it have been proved that the thermoelectric thin films have high ZT value, the performance of the thermoelectric device is not as good as expected if the thermoelectric thin films are directly adopted into the conventional structure of device. As shown in FIG. 2, the P-type thermoelectric thin film 201 and the N-type thermoelectric thin film 202 are generally about 10 nanometers to 100 micrometers thick, while the upper and lower substrates 221a/221b are much thicker. Thus, there is an extremely close distance (about the thickness of the thermoelectric thin films 201/202) between the hot side and the cold side of the conventional thermoelectric device of FIG. 2, and a heat flux quickly transferring in a short distance between the hot side and the cold side has an effect on the efficiency of the thermoelectric device. On the other words, it is not sufficient to sustain the temperature difference between the hot side and the cold side of the thermoelectric device in a meaningful time. Moreover, factors that affect the electrical resistance and heat transmission resistance of the interface between the metal layers and the thermoelectric thin films have become considerable. Joule's heating also affects the efficiency of thermoelectric device. Therefore, the performances of conventional thermoelectric structure inside which the thermoelectric thin films have been inserted are not as great as expected.